The problems associated with corrosion-induced deterioration of reinforced concrete structures are now well understood. Steel reinforcement has generally performed well over the years in concrete structures, such as bridges, buildings, parking structures, piers, and wharves, since the alkaline environment of concrete causes the surface of the steel to “passivate” such that it does not corrode. Unfortunately, since concrete is inherently somewhat porous, exposure to salt over a number of years results in the concrete becoming contaminated with chloride ions. Salt is commonly introduced in the form of seawater, set accelerators, or deicing salt.
When the chloride reaches the level of the reinforcing steel, and exceeds a certain threshold level for contamination, it destroys the ability of the concrete to keep the steel in a passive, non-corrosive state. It has been determined that a chloride concentration of 0.6 Kg per cubic meter of concrete is a critical value above which corrosion of the steel can occur. The products of corrosion of the steel occupy two and one-half to four times the volume of the original steel, and this expansion exerts a tremendous tensile force on the surrounding concrete. When this tensile force exceeds the tensile strength of the concrete, cracking and delaminations develop. With continued corrosion, freezing and thawing, and traffic pounding, the utility or integrity of the structure is finally compromised and repair or replacement becomes necessary. Reinforced concrete structures continue to deteriorate at an alarming rate. In a recent report to the United States Congress, the Federal Highway Administration reported that of the nation's 577,000 bridges, 266,000 (39% of the total) were classified as deficient, and that 134,000 (23% of the total) were classified as structurally deficient. Structurally deficient bridges are those that are closed, restricted to light vehicles only, or that require immediate rehabilitation to remain open. The damage on most of these bridges is caused by corrosion. The United States Department of Transportation has estimated that $90.9 billion will be needed to replace or repair the damage on these existing bridges.
Many solutions to this problem have been proposed, including higher quality concrete, improved construction practices, increased concrete cover over the reinforcing steel, specialty concretes, corrosion inhibiting admixtures, surface sealers, and electrochemical techniques, such as cathodic protection and chloride removal. Of these techniques, only cathodic protection is capable of controlling corrosion of reinforcing steel over an extended period of time without complete removal of the salt-contaminated concrete.
Cathodic protection reduces or eliminates corrosion of the steel by making it the cathode of an electrochemical cell. This results in cathodic polarization of the steel, which tends to suppress oxidation reactions (such as corrosion) in favor of reduction reactions (such as oxygen reduction). Cathodic protection was first applied to a reinforced concrete bridge deck in 1973. Since then, understanding and techniques have improved, and today cathodic protection has been applied to over one million square meters of concrete structure worldwide. Anodes, in particular, have been the subject of much attention, and several different types of anodes have evolved for specific circumstances and different types of structures.
The most commonly used type of cathodic protection system is impressed current cathodic protection (ICCP), which is characterized by the use of inert anodes, such as carbon, titanium suboxide and, most commonly, catalyzed titanium. This protection system also requires the use of an auxiliary power supply to cause protective current to flow through the circuit, along with attendant wiring and electrical conduit. This type of cathodic protection has been generally successful, but problems have been reported with reliability and maintenance of the power supply. Problems have also been reported relating to the durability of the anode itself, as well as the concrete immediately adjacent to the anode, since one of the products of reaction at an inert anode is acid (H+). Acid attacks the integrity of the cement paste phase within concrete. Finally, the complexity of ICCP systems requires additional monitoring and maintenance, which results in additional operating costs.
A second type of cathodic protection, known as galvanic cathodic protection (GCP), offers certain important advantages over ICCP. This galvanic cathode protection uses sacrificial anodes, such as zinc and aluminum, and alloys thereof, which have inherently negative electrochemical potentials. When such anodes are used, protective current flows in the circuit without need for an external power supply since the reactions that occur are thermodynamically favored. The system, therefore, requires no rectifier, external wiring or conduit. This simplicity increases reliability and reduces initial cost, as well as costs associated with long term monitoring and maintenance. Also, the use of GCP to protect high-strength prestressed steel from corrosion is considered inherently safe from the standpoint of hydrogen embrittlement. Recognizing these advantages, the Federal Highway Administration issued a Broad Agency Announcement (BAA) in 1992 for the study and development of sacrificial anode technology applied to reinforced and prestressed bridge components. As a result of this announcement and the technology that was developed because of this BAA, interest in GCP has greatly increased over the past few years.
In PCT Published Application WO94/29496 and in U.S. Pat. No. 6,022,469 by Page, a method of galvanic cathodic protection is disclosed wherein a zinc or zinc alloy anode is surrounded by a mortar containing an agent to maintain a high pH in the mortar surrounding the anode. This agent, specifically lithium hydroxide (LiOH), serves to prevent passivation of the zinc anode and maintain the anode in an electrochemically active state. In this method, the zinc anode is electrically attached to the reinforcing steel causing protective current to flow and mitigating subsequent corrosion of the steel.
In U.S. Pat. No. 5,292,411, Bartholomew et al disclose a method of patching an eroded area of concrete comprising the use of a metal anode having an ionically conductive hydrogel attached to at least a portion of the anode. In this patent, it is taught that the anode and the hydrogel are flexible and are conformed within the eroded area, the anode being in elongated foil form.
In U.S. patent application Ser. No. 08/839,292 filed on Apr. 17, 1997 by Bennett, the use of deliquescent or hygroscopic chemicals, collectively called “humectants” is disclosed to maintain a galvanic sprayed zinc anode in an active state and delivering protective current. In U.S. Pat. No. 6,033,553, two of the most effective such chemicals, namely lithium nitrate and lithium bromide (LiNO3 and LiBr), are disclosed to enhance the performance of sprayed zinc anodes. And in U.S. Pat. No. 6,217,742 B1, issued Apr. 17, 2001, Bennett discloses the use of LiNO3 and LiBr to enhance the performance of embedded discrete anodes. And finally, in U.S. Pat. No. 6,165,346, issued Dec. 26, 2000, Whitmore broadly claims the use of deliquescent chemicals to enhance the performance of the apparatus disclosed by Page in U.S. Pat. No. 6,022,469.
In PCT application Serial No. PCT/US02/30030, filed Sep. 20, 2002, a method of cathodic protection of reinforcing steel is disclosed comprising a sacrificial anode embedded in an ionically conductive compressible matrix designed to absorb the expansive products of corrosion of the sacrificial anode metal.
In U.S. Pat. No. 6,572,760 B2, issued Jun. 3, 2003, Whitmore discloses the use of a deliquescent material bound into a porous anode body, which acts to maintain the anode electrochemically active, while providing room for the expansive products of corrosion. The same patent discloses several mechanical means of making electrical connection to the reinforcing steel within a hole drilled into the concrete covering material. Many of these means involve driven pins, impact tools, and other specialized techniques. These techniques are all relatively complex and difficult to perform.
Finally, in U.S. Pat. No. 6,193,857, issued Feb. 27, 2001, Davison et al describe an anode assembly comprising a block of anode material cast around an elongated electrical connector (wire). Contact is made between the elongated connector and the reinforcing steel by winding the connector around the reinforcing steel and twisting the ends of the connector together using a twisting tool. This form of connection is simpler, and easier to execute than those of Whitmore, but is still laborious and time-consuming on site.
The anodes described above and the means of connection disclosed have become the basis for commercial products designed to extend the life of patch repair and to cathodically protect reinforced concrete structures from corrosion. But the configuration of the devices currently sold is not convenient for installation in actual patch repair. The commercial devices measure 2.5 inches (64 mm) in diameter by 1.25 inches (32 mm) thick, and are intended to mount against exposed reinforcing steel in patch repair. Installation of a device with this configuration does not conform well to established specifications for concrete repair. For example, Ohio Department of Transportation (ODOT) TS-519 specifications require a minimum of 1.25 inches (32 mm) of concrete cover over reinforcing bars, and excavation of concrete to 0.75 inch (19 mm) behind reinforcing bars. If the device currently sold is mounted against a reinforcing bar in vertical configuration, then the top of the device will be exposed if the concrete cover is minimum. On the other hand, if the device is mounted against and beneath the reinforcing bar in horizontal configuration, this will require the installer to chip out at least an additional 0.375 inch (10 mm) behind the bar to make room for the device, and even then patch concrete will not completely encapsulate the device unless even more concrete is removed. This results in considerable additional installation expense.
Mounting the device currently sold directly against the reinforcing bar creates another serious problem. Protective current will tend to flow to the reinforcing bars where the resistance path is lowest, and so a large portion of the current will “dump” directly to the bar against which the device is mounted. This diminishes protective current flow to the reinforcing steel outside the patch, where current and protection are more needed. It also has the effect of shortening anode life, since it causes total current to increase needlessly. This problem is sometimes averted in the field by coating the steel where the device is mounted with non-conductive epoxy, but this process is time consuming and messy, and is seldom used.